1. Field of the Invention
The present invention relates in general to self-emulsifying epoxy resins that are useful in dispersions suitable for: coatings applications; matt clear-coats having good adhesion to a wide variety of substrates, high hardness and good resistance properties; electrodeposition coatings; water-dilutable adhesives; and as additives to polymer cements.
2. Description of Related Art
It is known to prepare stable, aqueous dispersions of synthetic resins by emulsifying corresponding monomers or oligomers in an aqueous medium, using appropriate auxiliaries (emulsifiers), and then carrying out a polymerization reaction. By using this known reaction sequence, it is possible, for example, to prepare aqueous dispersions of acrylate resins by emulsion polymerization. In the case of condensation products, such as epoxy resins, which are difficult or even impossible to prepare by emulsion condensation, it is necessary, however, to prepare aqueous dispersions by first synthesizing the resin and then dispersing it in water. Such secondary dispersions require greater quantities of emulsifier, separate after only a short time and in general also display poor film-forming properties.
EP-A 0 051 483 describes and claims epoxy resin dispersions of self-emulsifying epoxy resins, which comprise polyalkylene glycol glycidyl ethers and also, if desired, a monoepoxide as reactive diluent. The maximum particle size of this dispersion is described as being about 3 .mu.m. Films produced from these dispersions and suitable curing agents possess, owing to the content of the relatively slow-reacting polyalkylene glycol glycidyl ethers and, if appropriate, monoepoxides which act as chain terminators, a relatively soft surface which restricts the utility of the epoxy dispersions described in this document.
U.S. Pat. No. 4,423,201 describes the preparation of self-emulsifying epoxy resins from diglycidyl ethers of aromatic polyols, from aromatic polyols and from reaction products of long-chain aliphatic polyether glycols with diisocyanates and aromatic polyols. The dispersion of these resins in water, however, is possible only with the aid of relatively large quantities of organic solvents, and in addition the particle sizes obtained are relatively high. The coatings produced using these epoxy resin dispersions, moreover, are relatively soft.
In Patent EP-B 0 272 595, proposals have already been made for aqueous dispersions based on specific self-emulsifying epoxy resins, which couple good storage stability with a low content of organic solvents and produce coatings having good surface properties. The coatings produced with these dispersions, however, have a tendency after a long time to become brittle, thereby adversely affecting technological properties such as, for example, elasticity or the anticorrosion effect.
EP-A 0 530 602 describes improved variants (based on flexibilized epoxy resins) of system 1, which are outstandingly suitable for aqueous anticorrosion coatings and can be prepared almost or even completely without solvents. A disadvantage of these dispersions, however, is their low shear stability, since they are externally emulsified systems.
For both of the above-mentioned systems, a specific emulsifier is obtained by condensing an aliphatic polyol having a number-average molar mass M.sub.n of from 200 to 20,000 g/mol with an epoxide compound having at least two epoxide groups per molecule and an epoxide group content of from 10,000 to 500 mmol/kg ("epoxide equivalent weight" EEW of from 100 to 2000 g/mol). In this system, the ratio of the number of OH groups to the number of epoxide groups is from 1:0.85 to 1:3.5 and the epoxide group content of this condensation product is between 200 and 2.5 mmol/kg (EEW 5000 to 400,000 g/mol). The synthesis of these products requires careful reaction control and also reactive catalysts, residues from which remain in the aqueous dispersion and may impair its long-term storage stability.
In the first case, this emulsifier is already present during resin synthesis and is evidently incorporated to a certain degree, by the slow-reacting secondary OH groups, into the epoxy resin. This system can therefore be referred to as being self-emulsifynig, and the expression "self-emulsifying" as it is used throughout this description, will denote this type of system. In the second case, the emulsifier is mixed in after resin synthesis, i.e. the system is externally emulsified. This difference manifests itself in particular in the shear stability, which for the second system is markedly poorer than for the first In both cases, however, the advantages of nonionic stabilization are evident, for example, in lack of sensitivity to electrolytes and in high storage stability.
The addition of solvents is necessary for technical reasons related on the one hand to the process, and related on the other hand to practical application. The "water hill" which occurs during phase inversion, i.e. the very high viscosity maximum, must be lowered by diluent additives, generally appropriate solvents, or by flexibilization of the resin in order for dispersion to succeed. For systems which cure at room temperature, moreover, the solvent acts as a coalescence agent to ensure flawless film formation.
The latter dispersions, especially those of EP-A 0 530 602, already have a markedly reduced solvent content with improved practical use properties. In the context of the ever more pressing need to minimize volatile organic components (VOCs) in aqueous coating compositions, however, even the reduced solvent content is a problem. Moreover, flexibilization of the base resin requires the use of expensive raw materials, which may in addition have adverse effects on the set of properties.
Systems which cure at room temperature, such as those for preventing corrosion and protecting buildings, or those which are subjected to forced drying, must be viewed particularly critically from the standpoint of solvent emission, since with these systems it is not possible, as in stoving units, to carry out thermal incineration of the solvent. A further improvement to the existing systems would, therefore, include a binder based on a completely solvent-free dispersion which could be used for room temperature-curing coatings.
From the viewpoint of practical use, the solvents employed serve predominantly as coalescence agents, which evaporate slowly from the film after it is formed, thereby giving rise to conditions unfavorable from the standpoint of occupational health and to the unwanted emission of organic constituents into the atmosphere. In addition, it is inevitable that solvent residues remain in the coating, and have an adverse effect on its properties. Without such solvents, however, the result is poor film formation and poor leveling, leading to turbid, poorly adhering films.
The application DE-A 43 27 493 describes a process by which it is possible to produce completely solvent-free dispersions. Instead of adding a solvent, free-radically polymerizable monomers (for example (meth)acrylates, styrene (derivatives), vinyl compounds, etc.) are added to the base resin and dispersion is then carried out in a customary manner. Once the dispersion has been obtained, emulsion polymerization is initiated in order to obtain by this means a completely solvent-free epoxy resin dispersion with a certain polymer dispersion component. These systems are of particular interest for stoving applications, since they show outstanding leveling, without addition of solvent, on curing with acids, melamine resins, etc.
That reactive diluents can have a positive effect (reducing viscosity) in conventional epoxy resin formulations and water-dilutable liquid resins is known to the person skilled in the art. Virtually no description has been given, however, of their use in dispersions of solid epoxy resin. U.S. Pat. Nos. 4,315,044, 4,399,242 and 4,608,406 describe the use of C.sub.8 -C.sub.80 -alkyl monoepoxides in nonionically stabilized dispersions of solid epoxy resins, for curing at room temperature or with heating. In these patents, the reactive diluents are stirred into the finished dispersion where they improve the shear stability, the freeze-thaw stability, the storage stability and the gloss of the coatings, although this can only be achieved satisfactorily by the additional use, in some cases, of glycols or glycol ethers. In contrast, nothing is said about the function of the reactive diluents as coalescence agents, with no mention in particular to the fact that the addition of the reactive diluent to the resin prior to the actual dispersion process is particularly advantageous.
The subsequent admixing of substances to a finished, nonionically stabilized dispersion of solid resin, especially when the dispersions have a high solids content by mass, is highly laborious, since it is necessary to carry out thorough homogenization of components of different viscosities and different polarities with one another, a process which cannot be achieved satisfactorily in relatively large mixing units. Furthermore, it is not possible in this way to achieve such a high degree of mixing of resin and reactive diluents, since these components are in separate micelles.
Reactive diluents can therefore be used in order to control the practical use properties of the dispersion, for example reducing the minium film-forming temperature, extending the processing time, improving gloss, shear stability and stability to freeze-thaw cycles, and exerting controlled influence over hardness and elasticity. Their advantage over the use of organic solvents is that these reactive diluents are incorporated into the film in the course of the curing reaction and therefore do not lead to the unwanted emission of organic components, thereby meeting the requirement for a low- or no-solvent system. Adding the reactive diluent before the dispersion process has the substantial advantage that with little effort it is much better emulsified than in the case of subsequent addition to the finished dispersion, leading to enhanced coating quality. In this case, reactive diluents and the base resin are in joint micelles, which cannot be achieved by subsequent, homogeneous incorporation of the reactive diluents. The content of reactive diluent can be apportioned to the solids content of the binder or coating material.
Thus, there exists a need to develop epoxy resins and self-emulsifying dispersions prepared therefrom whose emulsifying component is simple to prepare. There also exists a need to prepare these systems having particularly low-solvent contents, if desired, which can be achieved by use of reactive diluents.